How do I reinstall Snow Leopard on a 2007 iMac G5 with erased hard drive?

The only way to reinstall Snow Leopard is via the disks that came with the iMac. If you still have them, boot from the first disk and do the reinstall. If you no longer have the disks you may be able to find a set at one of the online stores. They are readily available; I just did a quick search and even found a factory sealed set for only $35.

If you have a second Mac you may be able to download the El Capitan installer and create a bootable flash drive to install in on your iMac G5.

How does an Intel build of El Capitan run on a PowerPC G5 Mac?

The last iMac G5 shipped with OS X Tiger 10.4.2 and the maximum installable version was OS X Leopard 10.5.8. How anyone managed to get Snow Leopard 10.6.* installed on this machine may have been a kludge at best. The last iMac G5 was discontinued in January 2006 contradicting the OP's hardware 2007 G5 statement.

EL CApitan is OS X 10.11, not 10.6.8, which is the final upgrade version of Snow Leopard. (10.6.3 is the last version of Snow Leopard on retail DVD).

I did a bit further research. The Late Fall 2007 iMacs were Intel, so the OP was mistaken calling it a G5. I had focussed on '2007' and 'Snow Leopard,' and missed the reference to a G5. The Late Fall 2007 iMacs were the first aluminum case iMacs and came with Tiger preinstalled but could be upgraded to El Capitan.

Had I really noticed the G5 reference I might have asked the OP to clarify exactly what the machine was instead of replying the way I did. Just goes to show one really has to stop & think things through before replying!

Interesting note, the immediate predecessor model was the January 2006 iMac, which had the same white polycarbonate case as the iMac G5!

Oops.

Have I been April Fooled by the OP? Or did I just not do my homework?

😉